It is known from U.S. Pat. No. 5,902,499 (Synova) how a laser beam with sufficient high energy for ablating material (particularly for cutting different types of materials) can be coupled into a water jet (or into any jet of an appropriate transparent liquid medium). The laser light is optically guided in the water jet in the same way as a light beam in an optical fibre and impinges on the work piece with an effective diameter substantially corresponding to the diameter of the water jet.
The main elements of the so called LWJ® technology consist in a lens system focussing the laser beam, a relatively thin high-pressure water chamber (also called coupling unit) and a nozzle with a small diameter. A typical nozzle diameter is 20 to 150 micrometers. In addition to its guiding properties, the water jet serves as a cooling medium for the machined part of the work piece and removes in a very efficient way the ablated material, leading to very good machining quality.
It is known from WO 2006/050622 (Synova) that the stability of the water jet plays a crucial role for achieving a good quality cut and an efficient machining process. One important characteristic of the water jet is its coherence length i.e. the travel distance in which the jet remains laminar. This is also the region where the water jet has its best light guiding properties. Micromachining is possible as long as the work piece is located between the nozzle and the coherence length. This distance is called working distance.
The working distance is known to be about 1000 times the nozzle diameter. Since the nozzles diameter is in the range of 20 to 150 micrometers, the working distance varies from about 20 mm to 150 mm. Although such distances are well suited for most machining processes, there are some specific applications, which require the placement of the workpiece close to the nozzle. From a theoretical point of view, there are no limitations regarding the minimum distance from the nozzle to the workpiece. However, experience shows that sometimes the following phenomena occur:                1. Perturbation of the water jet stability due to feedback effects as a result of interaction with the work piece. The perturbation of the water jet can be partially associated with the generation of surface waves (oscillations). These oscillations can propagate in the vertical direction, toward the nozzle. It may even reach the nozzle orifice.        2. Suction of the ablated material and its deposition on the backside of the nozzle. The impinging water jet has very high speed, which creates a de-pressurization around the jet, which leads to suction of the particles from the ablated material. The particles from the ablated material may also reach the nozzle orifice region.        3. Back reflected laser and plasma light, during the machining process of the work piece.        
Due to these phenomena the lifetime of the nozzle often is reduced compared to longer working distances and that the water jet stability is less than expected for short working distances. These effects are particularly a problem when a laser beam with high optical power is required.